In DEP (Direct Electrostatic Printing) the toner or developing material is deposited directly in an image-wise way on a receiving substrate, the latter not bearing any image-wise latent electrostatic image. In the case that the substrate is an intermediate endless flexible belt (e.g. aluminium, polyimide etc.), the image-wise deposited toner must be transferred onto another final substrate. If, however, the toner is deposited directly on the final receiving substrate, a possibility is fulfilled to create directly the image on the final receiving substrate, e.g. plain paper, transparency, etc. This deposition step is followed by a final fusing step.
This makes the method different from classical electrography, in which a latent electrostatic image on a charge retentive surface is developed by a suitable material to make the latent image visible. Further on, either the powder image is fused directly to said charge retentive surface, which then results in a direct electrographic print, or the powder image is subsequently transferred to the final substrate and then fused to that medium. The latter process results in an indirect electrographic print. The final substrate may be a transparent medium, opaque polymeric film, paper, etc.
DEP is also markedly different from electrophotography in which an additional step and additional member is introduced to create the latent electrostatic image. More specifically, a photoconductor is used and a charging/exposure cycle is necessary.
A DEP device is disclosed in e.g. U.S. Pat. No. 3,689,935. This document discloses an electrostatic line printer having a multi-layered particle modulator or printhead structure comprising:
a layer of insulating material, called isolation layer; PA1 a shield electrode consisting of a continuous layer of conductive material on one side of the isolation layer; PA1 a plurality of control electrodes formed by a segmented layer of conductive material on the other side of the isolation layer; and PA1 at least one row of apertures. PA1 creating a flow of charged toner particles in an electrical field from a magnetic brush to a substrate, PA1 image wise modulating said flow of charged toner particles by a printhead structure comprising printing apertures and control electrodes, PA1 image wise depositing toner particles, from said image wise modulated flow of charged toner particles, on said substrate and PA1 fixing said toner particles to said substrate, characterised in that PA1 said flow of charged toner particles is created directly from said magnetic brush, carrying carrier particles and toner particles and PA1 said carrier particles have a specific volume resistivity between 10.sup.1 .OMEGA..multidot.cm and 10.sup.9 .OMEGA..multidot.cm and PA1 said carrier particles have a specific density lower than 5 g/cm.sup.3.
Each control electrode is formed around one aperture and is isolated from each other control electrode.
Selected potentials are applied to each of the control electrodes while a fixed potential is applied to the shield electrode. An overall applied propulsion field between a toner delivery means (this wording is throughout this document to indicate the means for delivering toner particles) and a receiving member support projects charged toner particles through a row of apertures of the printhead structure. The intensity of the particle stream is modulated according to the pattern of potentials applied to the control electrodes. The modulated stream of charged particles impinges upon a receiving member substrate, interposed in the modulated particle stream. The receiving member substrate is transported in a direction orthogonal to the printhead structure, to provide a line-by-line scan printing. The shield electrode may face the toner delivery means and the control electrode may face the receiving member substrate. A DC field is applied between the printhead structure and a single back electrode on the receiving member support. This propulsion field is responsible for the attraction of toner to the receiving member substrate that is placed between the printhead structure and the back electrode. The printhead structure as described in U.S. Pat. No. 3,689,935 suffers from the fact that high speed printing at high printing quality is limited by clogging of some apertures. By implementing a large number of rows of printing apertures, the overall printing speed can, in theory, be enhanced, but in practice it is found that said printing speed is levelled to the amount of toner that can pass through the row of apertures having the smallest flux of toner particles, and banding with a frequency corresponding to the different rows of printing apertures can be easily observed.
The problem of obtaining a different printing density for different rows of printing apertures, i.e. banding, has been tackled in various ways. In U.S. Pat. No. 5,214,451 a printhead structure has been described consisting of different rows of apertures, each having a different shield electrode segment. During printing the voltage applied to the different shield electrode segments corresponding to the different rows of printing apertures is changed, so that these apertures that are located at a larger distance from the toner application module are tuned for a larger electrostatic propulsion field from said toner application module towards said back electrode structure, resulting in enhanced density profiles with less banding. The toner flux can be slightly enhanced for the "low density" rows of printing apertures, said toner flux must be greatly reduced for the "high density" rows of printing apertures. As a result the overall printing speed is reduced if enhanced image quality regarding white banding is preferred.
In U.S. Pat. No. 5,040,004 a moving belt is introduced as toner application module, said moving belt sliding over an accurately positioned shoe that is placed at close distance from said printhead structure. With this design the distance, and as a consequence also the propulsion field, can be finely tuned to be equal for all rows of printing apertures. This, however, causes very accurate and expensive means to be used in order to fabricate a toner application module according to said invention. Moreover, sliding contact, is never beneficial for excellent long term stability and reliability.
In U.S. Pat. No. 5,327,169, EP-A-675 417, JP-A-60 263 962 and JP-A-08 058142 a magnetic brush using a two-component development system has been described as toner application module in a DEP device. Also in DEP devices using a magnetic brush as toner delivery means, the distance between the magnetic brush and different sets of rows of printing apertures is different. Due to this fact also the propulsion field wherein the toner particles are jetted to the substrate is different for different sets of rows of printing apertures and thus the problem of banding still exists.
According to EP-A-731 394, in a DEP device using a magnetic brush as toner delivery means, the curvature of said magnetic brush is adapted to the extension in said printhead structure, so that banding can be minimised. This concept, however, leads to the introduction of more expensive magnetic brush devices, making the DEP device considerably more complicated, weighty and expensive.
In DE-A-195 34 705 the toner application device and printhead structure has been implemented twice for each colour, so that the problem of white banding can be reduced, but again at the expense of complexity and cost of the printing device.
In CA-A-2 135 705 a toner application device similar to a video cassette is used. A flexible band carrying toner particles is moved in a direction orthogonal to the rows of printing apertures, yielding a constant distance, and propulsion field, for every row of printing apertures. The main drawback of this system, however, is the consumption of toner particles from one side of a row of printing apertures to the other side, making it not possible to print with an equal density profile over the complete width of the receiver material.
In EP-A-587 366 the printhead structure is bent over the roller-shaped toner applicator so that for every row of printing apertures the distance towards the toner application module (and thus the propulsion field) is rather constant. The main drawback of this device is again the frictional contact over toner particles that greatly reduces the overall printing quality and long term stability.
When the cylindrical toner delivery means, from which the toner particles are extracted and brought in the neighbourhood of a printhead structure, is not a perfectly centred cylinder, white banding orthogonal to the printing direction can be observed. This banding can be attributed to fluctuations in distance between the surface of the toner delivery means and the printhead structure. In EP-A-736 822 it is described to minimise said banding by adjusting the rotation speed of the cylindrical toner delivery means to the speed by which the receiving substrate is moved. In order to keep the printing speed high it is necessary that the cylindrical toner delivery means is rotated at very high speed, which can bring problems with wear, mechanical stability, etc.
There is thus still a need for a DEP device yielding images of high density without white banding in a reliable, long-lasting way and without the need for mechanically complex means to diminish the possibility of banding.